Validation of water vapor measured by SABER on the TIMED satellite

Abstract

The Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) measured water vapor (H2O) had errors of unknown origin that were recently identified and corrected. The cause of the errors was determined to be unaccounted for spectral out-of-band (OOB) radiance in the H2O channel centered at 6.8 μm arising from ozone emission in the 9.6 μm band. The corrected SABER radiance profiles have been used to produce a long-term H2O data base labeled as version 2.07 (v2.07). Water vapor volume mixing ratio (VMR) vertical profiles are available in the SABER data archive covering the stratosphere and mesosphere extending from near the tropopause at ~100 hPa (~16 km) to the mesopause region at ~ 0.006 hPa (~83 km), and over the time period from 25 January 2002 to the present day. The random error of the v2.07 product is smaller than 4% at 60 km and below, while above this altitude it rapidly increases to 30% (at 80 km), mainly due to low signal-to-noise. The estimated systematic error of SABER v2.07 H2O is about 10–20%. Coincidence analysis between SABER v2.07, MLS v4.2, ACE v3.5-3.6, MIPAS ESA reprocessed v6, and SOFIE v1.3 shows overall excellent agreement in the mean profile with the mean difference being within ±10% in most cases. In the stratopause region SABER H2O tends to be biased high relative to each of the other data sets used for comparisons especially in the SH polar winter where the mean difference reaches 20% or greater. In polar summer above 80 km, SABER H2O is biased low by ~20% compared to the other measurements. SABER H2O therefore reflects the polar winter and spring descent very well but in the summer PMC region the enhancement is weaker than expected. SABER H2O long-term series in the latitude range 50°S-50°N shows close agreement with MLS on a series of pressure levels throughout the stratosphere and mesosphere on inter-annual to decadal time scales. On these time scales also, throughout the years 2002–2005, SABER and MIPAS long-term time series agree well in the equatorial region which serves as an unprecedented validation for this time period. SABER H2O also captures the “tape recorder” phenomenon in the tropical tropopause region very well.